1. Field of the Invention
The present invention relates to a method for manufacturing pre-expanded particles for expansion molding in a mold (hereinafter referred to as "expansion molding").
2. Description of the Prior Art
The expansion molding by the use of pre-expanded particles of polyolefin such as polypropylene has hitherto been used widely. There have been proposed a number of techniques relating to the shape of such pre-expanded particles, but most of them relate to the method for manufacturing spherical pre-expanded particles. As to the method for manufacturing spherical pre-expanded particles, there is disclosed in e.g. Japanese Laid-open Patent Publication 41777/77 a method for suspending ethylene resin particles in an aqueous solution and then heating the suspension to a temperature above the melting point of the resin so as to manufacture spherical particles. This method, however, had a problem of requiring an additional step for sphering the resin particles, this resulting in a rise of the manufacturing cost. In the expansion molding using pre-expanded beads, it is possible to improve the charging-in-a-mold property by making the pre-expanded particles as uniform and spherical as possible. If charging into the mold cannot be done properly, molded articles obtained by heating and subsequent cooling become worse in physical properties such as external appearance and dimensional shrinkage rate with respect to the mold, restoration rate from compression-induced strain, this resulting in failure of obtaining a satisfactory molded articles. Hence, there has been a strong need for a method for obtaining uniform and spherical pre-expanded particles easily at a moderate cost.